With the increase in digital communications between geostationary satellites and various forms of mobile platforms, such as high speed aircraft, the need for an optimized, physically small, lightweight, low power, mechanically scanned antenna structure has grown in importance. In applications where such a mechanically scanned antenna system needs to be located on the external surface of a high speed mobile platform such as a jet aircraft, the need for a lightweight antenna system that is also compact and that can be mechanically scanned about both azimuth and elevation axes, with low power and within a small swept volume, is especially important. The heavier the apparatus, the greater are the forces applied to the external surface of the aircraft, and the costlier is the structural reinforcement required for installation. The heavier the mechanically rotating sections of the apparatus the greater the motor drive power required for rotation. The added weight of the heavier apparatus, structural reinforcement and rotating components contribute to losses in fuel economy and reduction in the prime power of the mobile platforms. Thus, it should be apparent that any structure that allows for supporting the aperture so that the overall swept volume of the aperture can be minimized by an amount X, will reduce the height and footprint of the radome that needs to be used to cover the aperture by a corresponding amount.
Weight is an especially important factor for a mechanically scanned antenna aperture used on mobile platforms. This is especially true on high speed mobile platforms such as military and commercial aircraft. Minimizing the weight of the aperture and its associated supporting structure, without reducing the strength and robustness of the aperture and its supporting structure, is highly desirable because it minimizes the adverse effect on fuel economy that the aperture could otherwise produce.
Another important factor for a mechanically scanned system on a mobile platform is to minimize the size of the antenna aperture. The smaller the antenna aperture, the smaller is the swept volume and the radome needed to cover the aperture. The less the aerodynamic drag on the small mobile platform, the lower the fuel costs will be for operating the vehicle. Another consideration is that the antenna aperture size is part of the transmit function's effective isotropic radiated power (EIRP) and the receiver function's gain over temperature (G/T). RF losses degrade both EIRP and G/T in communications between mobile platforms near the earth and Ku- and Ka-band satellites in distant geostationary orbits. Minimizing RF losses helps to promote smaller antenna apertures, smaller radomes and produce smaller aerodynamic drag.
Another important factor for a mechanically scanned system on a mobile platform is minimizing the power required for the motors, which drive the mechanically scanned system (reflector, sub-reflector, waveguide, components, structure, etc.) about the elevation and azimuth axes. The smaller and lighter the aperture structure is, the more likely that less powerful motors can be implemented.
With brief reference to FIGS. 1 and 2, the swept volume consideration is illustrated. FIG. 1 illustrates a mechanically scanned aperture that is rotated about a pivot point “P”, when viewing the aperture from a plan or top view. The dashed line “D” represents the minimum swept area that is required in the azimuth plane for the aperture to be rotated 360°. The closer the pivot point “P” is to the back of the aperture, the smaller is the minimum swept volume. The azimuth axis of rotation is centered within a radio frequency (RF) rotary joint. The diameter of the RF rotary joint in the azimuth plane determines the minimum spacing between the pivot point and the antenna aperture and the ensuing minimum azimuth swept area. “A small diameter, compact coaxial or waveguide rotary joint is required to minimize the azimuth swept area. The bottom section of the rotary joint is stationary and the top section rotates. In practice, a 2-channel rotary joint is required to allow for the vertical and horizontal polarized RF signals to connect to the RF ports on the mechanically scanning antenna on top, and the fixed RF ports connected to the mobile platform supporting structure on the bottom. FIG. 2 illustrates the elevation axis pivot point PE. The aperture is shown in a vertical orientation in dashed lines, and the dashed line arc represents the swept volume required for scanning from a horizontal plane to a vertical plane about 360° of azimuth rotation. The height and depth of the apparatus, (aperture and attached components) must be optimized for minimum size, in addition to selecting the elevation rotational axis within the center of the apparatus in the elevation plane to minimize the vertical swept volume. FIG. 2 also illustrates that the structure supporting the aperture typically makes use of some form of supporting assembly that comprises a stationary member and a movable member positioned on top of the stationary member. The total thickness of these two elements also contributes to the swept volume required for the two aperture axes.